The present invention relates to a rotary airlock feeder for transferring particulate material from a storage chamber to a pneumatic convey line, and more particularly to such a feeder equipped with a low pressure air purge for preventing particulate material, dust, and debris from entering the bearings, seals, and critical moving parts of the feeder.
Rotary airlock feeders are commonly used in situations where particulate material must be transferred between a storage chamber, such as a hopper, and a pneumatic conveyor. A typical rotary airlock feeder includes a generally cylindrical housing connected transversely between the bottom of the storage chamber and the conveyor. Within the airlock feeder, a rotary shaft extends longitudinally through the cylindrical housing, supported on either end by a respective bearing positioned in a corresponding fixed end plate. A number of radially extending blades are attached to and extend outward from and run parallel to the shaft. A pair of rotary shrouds are also attached to the shaft, extending concentrically outward therefrom and perpendicular thereto, with the rotary shrouds abutting the blades, one shroud on either end. The rotary shrouds are positioned just inside of respective ones of the fixed end plates, allowing just enough of a cavity therebetween to permit free rotation of the rotary shrouds. The blades collectively form a number of annular wedge shaped spaces closed at either end by the rotary shrouds, which spaces rotate with the shaft. As the shaft is rotated, material is fed via these rotating wedge-shaped spaces from the top of the feeder to the bottom of the feeder and thence into the convey line. The rotating blades and the rotary shrouds act as an airlock to prevent pressurized air from being fed from the convey line directly into the storage chamber, which would disrupt the material flow into the convey line and disrupt air and material flow through the convey line itself.
While such rotary airlock feeders have been relatively successful in promoting the smooth flow of particulate material into pneumatic convey lines, they are subject to frequent maintenance and downtime for cleaning due to dust and debris from the particulate material entering the bearings, seals and other critical moving parts and clogging the cavities between the fixed and rotary shrouds. Also, due to very close machining tolerances between the rotating shrouds and the walls of the cylindrical housing, particulate material entering the gap between the shrouds and the housing walls causes rapid abrasive wear on both the shroud peripheries and the cylinder walls themselves.
Prior art attempts have been made to prevent particulate material, dust and debris from clogging airlock feeders. Often a flexible annular seal extends between each rotary shroud and the corresponding fixed end plate to prevent debris from entering the cavities between the fixed and rotary shrouds and to prevent the material from collecting in the gaps between the rotary shrouds and the cylindrical housing walls. However, these flexible seals repeatedly, temporarily fail as the shaft rotates, due to vibration of the feeder. Thus, minute quantities of debris repeatedly enter the cavities between the end plates, eventually migrating outward along the shaft and clogging the bearings, seals and the shaft itself, as well as eroding the rotary shroud peripheries and housing walls.
One common technique of preventing such repeated ingress of debris across the seals, as reflected by U.S. Pat. No. 3,151,784 to Tailor, is to introduce a gas under relatively high pressure into the fixed end plates about the periphery of the rotary shaft just inside the support bearings. This causes a pressure differential to exist between the shaft and the interior of the feeder. This pressure differential between the shaft and the airlock feeder interior theoretically forces the seal outward and keeps debris from entering the end plate spaces and clogging the shaft and bearings.
In practice, the volume of high pressure air generally used to pressurize the shafts, in systems such as the Tailor patent, is insufficient to purge the cavities between the fixed and the rotary shrouds or to prevent material from entering the gap between the rotary shrouds and the cylindrical housing. This can allow dust, debris and small particles of the particulate material to enter these cavities, eventually clogging the cavities and requiring the feeder to be frequently stopped and cleaned. Repeated migration of such material into the working components of such feeders can drastically reduce the useful life of the feeders. Furthermore, such systems require a dedicated source of high pressure air or gas in addition to the low pressure source used in the pneumatic conveyor. This can be a costly solution, particularly where it is desired to supply a sufficient volume of high pressure air to successfully purge the end plate cavities as well.
Accordingly, a need still exists for a simple and reliable system for preventing dust and debris or particulate material from clogging the shaft, seals, and bearings of a rotary airlock feeder as well as causing premature abrasive wear of the periperies of the rotary shrouds and housing walls. Such a system should also preferably prevent such debris from entering the cavities between the fixed and rotary shrouds in the feeder. In addition, the system should not require a costly and dedicated source of high pressure air separate from the pneumatic convey line air source.